Fuel cells are cells directly converting chemical energy of fuels into electric energy by electrochemically oxidizing hydrogen, methanol, and the like in cells, and then extracting the electric energy. They are therefore focused on as clean electric energy sources. In particular, polymer electrolyte fuel cells can drive at temperatures lower than other cells, and they are expected as alternative power sources for automobiles, household cogeneration systems, portable electric generators, and the like.
Such a polymer electrolyte fuel cell comprises at least a membrane electrode assembly. The membrane electrode assembly comprises an electrolyte membrane and gas diffusion electrodes. Each gas diffusion electrode is formed by stacking an electrode catalyst layer and a gas diffusion layer, and the gas diffusion electrodes are joined to the respective faces of the electrolyte membrane. The electrolyte membrane herein is a material having a strong acid group such as a sulfonic acid group and a carboxylic acid group in a polymer chain, and having proton-selective permeability. Examples of such an electrolyte membrane include perfluoro proton exchange membranes typically such as Nafion (registered trademark, Du Pont) having high chemical stability.
In order to drive a fuel cell, a fuel (e.g. hydrogen) is supplied to a gas diffusion electrode on the anode side, while an oxidant (e.g. oxygen or air) is supplied to a gas diffusion electrode on the cathode side, and both of the electrodes are coupled through an outside circuit. Thereby, the fuel cell operates. Specifically, in the case that hydrogen is used as the fuel, hydrogen is oxidized and generates a proton on an anode catalyst. This proton passes through an electrolyte binder in the anode catalyst layer, moves inside the electrolyte membrane, and reaches on a cathode catalyst through the electrolyte binder inside the cathode catalyst layer. On the other hand, an electron generated at the same time of the proton by oxidation of hydrogen reaches the gas diffusion electrode on the cathode side through the outside circuit. On the cathode catalyst, the proton and oxygen in the oxidant react to generate water. At this time, electric energy is generated.
Since polymer electrolyte fuel cells show a high energy conversion rate with a small environmental burden, they are expected as stationary cogeneration systems and vehicle-mounted power sources. In the automobile applications, fuel cells are generally driven at around 80° C. at the present time. In order to popularize fuel-cell vehicles, however, downsizing of radiators and simplification of humidifiers, and resulting cost reduction are required. For this purpose, an electrolyte membrane is demanded which is capable of being applied to driving under high-temperature and low-humidity conditions (corresponding to a driving temperature of 100° C. to 120° C. and a humidity of 20 to 50% RH), and which shows high performance under wide driving environments (room temperature to 120° C./20 to 100% RH). Specifically, as shown in Non-Patent Document 1, the proton conductivity is required to be 0.10 S/cm or higher at 50% RH for a driving temperature of 100° C., and the proton conductivity is required to be 0.10 S/cm or higher at 20% RH for a driving temperature of 120° C.
The conductivity of a conventional perfluoro proton exchange membrane, however, greatly depends on humidity, and it greatly decreases particularly at 50% RH or lower. Patent Documents 1 to 3 disclose a fluoroelectrolyte membrane having an equivalent weight (EW), that is, EW (g/eq) which is a dried weight per equivalent of a proton exchange group, of 670 to 776. As mentioned here, a reduction in the EW value, in other words, an increase in the capacity of the proton exchange leads to an increase in the conductivity. Further, Patent Document 4 discloses an electrolyte membrane which is less likely to be hydrothermally dissolved even at a low EW, and exemplifies an electrolyte membrane with an EW of 698. Patent Document 5 discloses one example of a method for producing a polymer electrolyte with an EW of 564.
In addition, perfluoro proton exchange membranes are known to deteriorate due to long-term use, and various stabilizing methods are proposed. For example, Patent Document 6 discloses a fluoropolymer electrolyte obtained through a polymerization step in which materials are copolymerized at a polymerization temperature of 0° C. to 35° C. using a radical polymerization initiator that comprises a fluorocompound having a molecular weight of 450 or higher.
Patent Document 1: JP 06-322034 A
Patent Document 2: JP 04-366137 A
Patent Document 3: WO 2002/096983
Patent Document 4: JP 2002-352819 A
Patent Document 5: JP 63-297406 A
Patent Document 6: JP 2006-173098 A
Non-Patent Document 1: H. Gasteiger and M. Mathias, In Proton Conducting Membrane Fuel Cells, P V 2002-31, pp. 1-22, The Electrochemical Society Proceedings Series (2002)